Constant-air-volume systems
O
Use in new systems discouraged by code.
+Q +P — Q +W +Q |
R |
|
0 |
/M |
———— XH1 |
Winter heating |
Summer:
Single mixing with room air:
O + R => M (cooling + dehumidification) => C (re-heat) => I (Q + W ) => R Double mixing with room air:
O + R => Ml (cooling + dehumidification) => C’ + R => M2 or I (Q + W ) => R Winter:
O (pre-heat) => HI + R => M (humidification) => H2 (re-heat) => I (Q + W) => R
4.3.1 Variable-air-volume systems
R |
Example:
You turn the fan speed up or down in your car. Q = m (iR — ii)
O
AHU fan varies power to match loads. Less load => lower fan power.
Pressure in supply ducts is maintained to a fixed value.
Design cooling:
• box is 100% open
• no reheat
Off-design cooling:
• zone temperature drops since cooling load decreases
• box throttles until minimum flow is reached
Dead band:
• no control action
• start reheat at lower limit
Off-design heating:
• minimum primary air
• thermostat increases reheat as space temperature falls
Design heating:
• fully energized
FIGURE 11.23 Vanable-air-volume system with optional reheat. |
VAV terminals:
• Single-blade dumper (pressure dependent or independent)
• Air valve
• Induction
Primary flow induces secondary flow from plenum.
Fully
|
|
(i) |
KIGl’RE 17.3 VAV boxes: fa) single-blade. pressure-dependent: (bl single-blade, pressure-»dependent:
:rvJ I Cl :l«r valve.
|
FIGURE 11.246 Schematic diagrams of series and parallel fan-powered VAV box. |
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Cutaway drawing of parallel VAV box. (Courtesy of Titus, inc.) |
{ (<*) |
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|
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(Optional) reheat Induction
FIGURE 11.24л Schematic diagram of induction VAV box. |
Fan — powered series: Fan is always on and space flow is constant. Damper controls supply of primary air. Perimeter zones may need baseboard or fan-coil units.
Fan-powered parallel: Fan injects plenum air to reheat. Supply pressure drives primary flow that is controlled by dumpers. Variable space flow => less fan energy.
Advantages of VAV: O
Disadvantages and problems of VAV for off design (low flow rate): O
Posted in Fundamentals of Heating. Ventilating, and Air-Conditioning